1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the invention relates to an exhaust gas purification device equipped with a NOx absorbing and reducing catalyst which absorbs NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean, and releases and purifies by reduction the absorbed NOx when the air-fuel ratio of the exhaust gas flowing in is rich.
2. Description of the Related Art
There has been known a NOx absorbing and reducing catalyst which absorbs NOx (nitrogen oxides) in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean, and releases and purifies, by reduction, the absorbed NOx when the air-fuel ratio of the exhaust gas flowing in becomes rich.
An exhaust gas purification device using the NOx absorbing and reducing catalyst of this type has been disclosed in, for example, Japanese Patent No. 2600492. In the exhaust gas purification device of the above patent, the NOx absorbing and reducing catalyst is disposed in the exhaust passage of an engine that operates at a lean air-fuel ratio. During a lean air-fuel ratio operation of the engine, NOx in the exhaust gas is absorbed by the NOx absorbing and reducing catalyst. When NOx is absorbed in an increased amount by the NOx absorbing and reducing catalyst, the rich spike operation is executed to operate the engine at an air-fuel ratio (or rich air-fuel ratio) smaller than the stoichiometric air-fuel ratio for a short period of time. Thus, the NOx that is absorbed is released from the NOx absorbing and reducing catalyst, and the released NOx is purified by reduction. That is, when the engine operating air-fuel ratio becomes rich, the oxygen concentration in the exhaust gas sharply drops compared with when the engine is operated at a lean air-fuel ratio, and the amounts of unburned HC and CO components sharply increase in the exhaust gas. Therefore, when the operating air-fuel ratio is changed over to a rich air-fuel ratio by the rich spike operation, NOx is released from the NOx absorbing and reducing catalyst and is reduced by being reacted with the unburned HC and CO components in the exhaust gas on the NOx absorbing and reducing catalyst.
The above-mentioned Japanese Patent No. 2600492 further discloses a constitution for purifying the HC and CO components emitted from the engine at the start of the engine by disposing a three-way catalyst in the exhaust passage on the upstream side of the NOx absorbing and reducing catalyst. The three-way catalyst of the above patent is disposed near the engine exhaust manifold through which the exhaust gas of a high temperature from the engine passes, and is heated to the activated temperature within a short period of time after the start of the engine. Therefore, HC and CO emitted in relatively large amounts from the engine are oxidized by the three-way catalyst after the engine started, and the quality of the exhaust gas, before the engine is warmed-up, is improved.
With the three-way catalyst being disposed in the exhaust passage on the upstream side of the NOx absorbing and reducing catalyst as taught in the above-mentioned Japanese Patent No. 2600492, it was considered that the ability of the NOx absorbing and reducing catalyst for purifying the exhaust gas often drops when the three-way catalyst possesses an O2 storage capability due to a delay in the change of the air-fuel ratio of the exhaust gas flowing into the NOx absorbing and reducing catalyst.
As is widely known, the three-way catalyst carries, as an additive, a metal component such as cerium Ce in addition to noble metal catalyst components such as platinum Pt, palladium Pd and rhodium Rh, so as to exhibit the O2 storage capability. That is, cerium carried as an additive by the catalyst bonds to oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst is higher than the stoichiometric air-fuel ratio (when the air-fuel ratio of the exhaust gas is lean) to form ceria (cerium oxide IV: CeO2) which stores oxygen. Further, when the air-fuel ratio of the exhaust gas flowing in is smaller than the stoichiometric air-fuel ratio (when the air-fuel ratio of the exhaust gas is rich), ceria releases oxygen and is transformed into cerium oxide III (Ce2O3); i.e., oxygen is released. Thus, the three-way catalyst having an O2 storage capability releases oxygen when the air-fuel ratio of the exhaust gas changes from the lean side to the rich side, and the air-fuel ratio of the exhaust gas that has passed through the three-way catalyst is maintained to be close to the stoichiometric air-fuel ratio, as long as oxygen is released from the three-way catalyst, even when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst has changed to the rich side.
However, when the three-way catalyst disposed in the exhaust passage on the upstream side of the NOx absorbing and reducing catalyst possesses an O2 storage capability, the exhaust gas flowing into the NOx absorbing and reducing catalyst does not readily acquire a rich air-fuel ratio but is temporarily maintained near the stoichiometric air-fuel ratio even when the air-fuel ratio of the exhaust gas from the engine has changed from the lean side to the rich side due to the rich spike operation of the engine. When the air-fuel ratio of the exhaust gas is changed from a lean air-fuel ratio to an air-fuel ratio close to the stoichiometric air-fuel ratio, NOx is released from the NOx absorbing and reducing catalyst. However, in this case, the air-fuel ratio of the exhaust gas is not rich enough, i.e., the exhaust gas is not containing HC and CO components in amounts sufficient for reducing all of NOx released and, hence, NOx that has not been reduced flows out to the downstream side of the NOx absorbing and reducing catalyst.
When the NOx absorbing and reducing catalyst was used, therefore, it was not considered desirable to dispose the three-way catalyst having an O2 storage capability in the exhaust passage on the upstream side of the NOx absorbing and reducing catalyst. When the three-way catalyst was disposed in the exhaust passage on the upstream side, therefore, it was considered that some countermeasure is required, for example, to remove cerium from the three-way catalyst, in order to lower the O2 storage capability.
According to the study conducted by the present inventors, however, it was found that when NOx is to be released from the NOx absorbing and reducing catalyst, the NOx absorbing and reducing catalyst exhibits enhanced performance for purifying NOx when the three-way catalyst or the like catalyst having O2 storage components is disposed at a position close to the NOx absorbing and reducing catalyst on the upstream side thereof. That is, when NOx is to be released from the NOx absorbing and reducing catalyst, the exhaust gas flowing into the catalyst must have a rich air-fuel ratio. In this case, when oxygen is released from the O2 storage components disposed at a position close to the NOx absorbing and reducing catalyst on the upstream side thereof, it has been found that NOx is released and reduced at a greatly increased rate.
It has not been clarified yet why the catalyst having O2 storage components disposed near the NOx absorbing and reducing catalyst on the upstream side thereof helps improve the performance of the NOx absorbing and reducing catalyst for purifying the exhaust gas. However, one of the causes is considered to be that, if the O2 storage components exist at a position close to the upstream side of the NOx absorbing and reducing catalyst when the exhaust gas of a rich air-fuel ratio is supplied, the HC and CO components in the exhaust gas are oxidized by the oxygen released from the O2 storage components and the temperature of the catalyst components on the NOx absorbing and reducing catalyst rises due to the heat of reaction. That is, one of the reasons is attributed to the release of NOx from the NOx absorbing and reducing catalyst promoted by the rise of temperature of the NOx absorbing and reducing catalyst components, whereby the catalytic activity is improved to improve the NOx purification ratio. Though this point will be described later in detail, there arises a problem in that the purifying performance of the NOx absorbing and reducing catalyst often cannot be utilized to a sufficient degree, contrary to the conventional opinion, if the O2 storage capability is eliminated as much as possible from the three-way catalyst or the like catalyst disposed on the upstream side of the NOx absorbing and reducing catalyst.
When the exhaust gas is rendered to possess a rich air-fuel ratio by executing the rich spike operation during the lean air-fuel ratio operation as taught in the above-mentioned Japanese Patent No. 2600492, the amounts of HC and CO components in the exhaust gas increase sharply. However, the HC and CO components tend to adhere on the catalyst components on the NOx absorbing and reducing catalyst. When the amounts of HC and CO components sharply increase in the exhaust gas flowing into the NOx absorbing and reducing catalyst, therefore, the surfaces of the catalyst components are covered with HC and CO. Therefore, the catalytic function drops, i.e., HC contamination and CO contamination take place, and the NOx absorbing and reducing catalyst exhibits decreased ability for purifying NOx.
The object of the present invention is to provide an exhaust gas purification device for an internal combustion engine capable of exhibiting a high NOx conversion capability by solving one or more of the above-mentioned problems.
The objects as set forth above are achieved by an exhaust gas purification device for an internal combustion engine, according to the present invention, comprising a NOx absorbing and reducing catalyst, which absorbs NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean and releases the absorbed NOx and purifies it by reduction when the air-fuel ratio of the exhaust gas flowing in is rich, disposed in an exhaust passage of the internal combustion engine, which is capable of selecting, as required, the operation at a lean air-fuel ratio and the operation at a rich air-fuel ratio, so that the NOx absorbing and reducing catalyst absorbs NOx in the exhaust gas when the engine is operated at a lean air-fuel ratio and that the absorbed NOx is released from the NOx absorbing and reducing catalyst and is purified by reduction when the engine is operated at a rich air-fuel ratio;
wherein oxygen storage components which absorb oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and release the absorbed oxygen when the air-fuel ratio of the exhaust gas is rich are carried by a substrate of the NOx absorbing and reducing catalyst on the upstream half portion of the substrate between an exhaust gas inlet side end to a central portion thereof.
According to this aspect of the invention, the oxygen storage components having O2 storage capability are carried by the upstream half portion of the NOx absorbing and reducing catalyst substrate. When the exhaust gas flowing into the NOx absorbing and reducing catalyst is rendered to possess a rich air-fuel ratio in order to release NOx from the NOx absorbing and reducing catalyst and to purify it by reduction, oxygen is released from the oxygen storage components and, at the same time, H2 and CO in the exhaust gas are oxidized with oxygen absorbed by the oxygen storage components on the surface of the substrate of the NOx absorbing and reducing catalyst, i.e., near the surfaces of the catalyst components of the NOx absorbing and reducing catalyst, whereby the temperature of the NOx absorbing and reducing catalyst components rises due to the heat of oxidation reaction. Accordingly, the NOx absorbing and reducing catalyst exhibits improved activity, and NOx is released at an increased rate from the NOx absorbing and reducing catalyst to enhance the efficiency for reducing the released NOx.
When the air-fuel ratio of the exhaust gas flowing into the NOx absorbing and reducing catalyst is lean, NOx in the exhaust gas flowing in is mainly absorbed by the NOx absorbing and reducing catalyst carried by the one-half (upstream half portion) of the catalyst substrate on the side of the inlet, and the amount of NOx absorption becomes greater in the upstream half portion than in the latter half portion. At the time of releasing NOx, therefore, NOx is mainly released from the upstream half portion of the substrate. With the oxygen storage components being carried by the upstream half portion of the carrier, therefore, the released NOx is efficiently purified by reduction in the whole device.
According to another aspect of the present invention, there is provided an exhaust gas purification device for an internal combustion engine, comprising a NOx absorbing and reducing catalyst which absorbs NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean and releases the absorbed NOx and purify it by reduction when the air-fuel ratio of the exhaust gas flowing in is rich disposed in an exhaust passage of the internal combustion engine which is capable of selecting, as required, the operation at a lean air-fuel ratio and the operation at a rich air-fuel ratio, so that the NOx absorbing and reducing catalyst absorbs NOx in the exhaust gas when the engine is operated at a lean air-fuel ratio and that the absorbed NOx is released from the NOx absorbing and reducing catalyst and is purified by reduction when the engine is operated at a rich air-fuel ratio;
wherein a three-way catalyst is disposed in the exhaust passage of the engine on the upstream side of the NOx absorbing and reducing catalyst and neighboring the NOx absorbing and reducing catalyst substrate.
According to this aspect of the invention, a separate three-way catalyst is disposed on the upstream side of the NOx absorbing and reducing catalyst and close thereto. When the engine is operated at a rich air-fuel ratio to release NOx from the NOx absorbing and reducing catalyst, the amounts of HC and CO components sharply increase in the exhaust gas flowing into the NOx absorbing and reducing catalyst. According to the present invention, however, the three-way catalyst is disposed at the position upstream of and close to the NOx absorbing and reducing catalyst. Therefore, the HC and CO components in the exhaust gas partly react on the three-way catalyst, and the HC and CO components flowing into NOx absorbing and reducing catalyst are prevented from sharply increasing. This suppresses the contamination of the NOx absorbing and reducing catalyst caused by an increase in the HC and CO components in the exhaust gas.
Further, the three-way catalyst creates the water-gas-shift reaction by which H2 is formed from CO and H2O under a rich air-fuel ratio condition. H2 is very strongly reductive and is more effective as a reducing agent for reducing NOx released from the NOx absorbing and reducing catalyst than HC and CO. Upon arranging the three-way catalyst on the upstream side of the NOx absorbing and reducing catalyst as is done in the present invention, therefore, it is allowed to decrease the amount of CO in the exhaust gas and to prevent the occurrence of contamination due to CO, as well as to efficiently reduce NOx that is released by supplying H2 to the NOx absorbing and reducing catalyst. Depending on the type of the engine, further, the HC component is contained in relatively large amounts in the exhaust gas during the lean air-fuel ratio operation and, hence, the NOx absorbing and reducing catalyst may often be contaminated even during the lean air-fuel ratio operation. Upon arranging the three-way catalyst on the upstream side of the NOx absorbing and reducing catalyst as described above, however, the occurrence of HC contamination is prevented while the engine is operating at a lean air-fuel ratio.
According to another aspect of the present invention, there is provided an exhaust gas purification device for an internal combustion engine, comprising a NOx absorbing and reducing catalyst which absorbs NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean and releases the absorbed NOx and purifies it by reduction when the air-fuel ratio of the exhaust gas flowing in is rich disposed in an exhaust passage of the internal combustion engine which is capable of selecting, as required, the operation at a lean air-fuel ratio and the operation at a rich air-fuel ratio, so that the NOx absorbing and reducing catalyst absorbs NOx in the exhaust gas when the engine is operated at a lean air-fuel ratio and that the absorbed NOx is released from the NOx absorbing and reducing catalyst and is purified by reduction when the engine is operated at a rich air-fuel ratio;
wherein the NOx absorbing and reducing catalyst includes a substrate, a NOx absorbing and reducing catalyst layer carrying NOx absorbing and reducing catalyst components on the substrate, and an oxygen storage component layer carrying oxygen storage components which absorb oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and release the absorbed oxygen when the air-fuel ratio of the exhaust gas is rich, the oxygen storage component layer being formed on the NOx absorbing and reducing catalyst layer.
According to this aspect of the invention, the NOx absorbing and reducing catalyst has a two-layer-coated structure including the NOx absorbing and reducing catalyst layer formed on the substrate, and the oxygen storage component layer formed on the upper side (i.e., on the exhaust side) of the NOx absorbing and reducing catalyst layer. When NOx is released from the NOx absorbing and reducing catalyst, therefore, heat produced by the reaction of CO and the like in the exhaust gas in the oxygen storage component layer is directly transmitted to the NOx absorbing and reducing catalyst components, whereby the temperature of the NOx absorbing and reducing catalyst components is efficiently raised.